The convergence of the media industry with the Internet and the appearance of mobile devices with rich media playing capabilities have created a variety of new services. In particular, nowadays users can consume, buy, share, and review media items in many new exciting ways. Yet, the variety and volume of media items is overwhelming. As a result, users may find it difficult to navigate through the vast variety of new media items and get what they like. To overcome this explosive content volume, helping users find items they like, many new recommendation services have emerged in recent years.